Heart rate is the speed of the heartbeat measured by a number of poundings per unit of time—typically beats-per-minute (“BPM”). In healthcare, fitness training, and stress management as well as in emerging emotion-aware and affect-aware user interfaces, heart rate is a useful physiological signal. Conventional techniques for monitoring heart rate typically use dedicated sensors, such as electrocardiogram (“ECG”) sensors and blood oxygen sensors (e.g., oximeters), embedded in specialized devices having form factors such as wristbands, watches, and chest bands. Such conventional deployments, however, are rife with inconveniences to users. Associated with these conventional techniques, for example, are costs of the devices, discomfort from wearing them, or effort required to employ them. As a result of these inconveniences, heart rates simply are not monitored, analyzed, or used in the normal course of most people's lives.